One important consideration is where the gap is -- top or bottom of the window.
If the cat can get their head through a small gap (and some cats -- including my little girl --can) but can't get their body through, they can get stuck and panic. If the gap is at the bottom, they could still injure themselves but I'd be more worried about a gap at the top where they could actually hang themselves.
Is something like this Tilting Window Protection available where you live and suitable for your window? Or Cat nets ?
I'll add that a cat doesn't need to be particularly small to escape through a small gap by an open window, and they don't always have good sense: I've had a medium size cat (who objected to being shut indoors overnight for his own safety) squeeze out through a narrow gap at the top of a window one floor up, and jump down onto a shed roof (lucky it was there) before descending to the ground. And another (who had two broken front legs in plaster after jumping out of a window one floor up) repeat the experiment though an-only-just-open window (and break both legs again).